On Macintosh, using OS X 10.11, I sometimes get many blank windows (windows with only a title bar but no content). These windows don't behave properly. What is the problem?
We don't know for sure what causes this but this problem is specific to OS X 10.11 (and possibly 10.10). Typically this happens when you switch from Igor to another application (either by hiding Igor using Cmd+H or by clicking on the desktop or the Dock icon for another application). We know that having had the Igor Debugger visible can result in this behavior, but we aren't certain that the Debugger is required. Sometimes Igor also crashes after being in this state, so if you get into this state it is best to save your experiment and restart Igor.
The only certain workaround we know for this problem is to upgrade your OS to macOS 10.12 (Sierra) or later.
Update (April 24, 2019): We have one report of a very similar problem (including a crash) from a user running Igor Pro 8.03 on macOS 10.14.4 (Mojave).
Igor Pro 8
Igor XOP Toolkit
Igor NIDAQ Tools MX